Abstract

Pyramidal neurons in superficial layers of cerebral
cortex have extensive horizontal axons that provide a substrate
for lateral interactions across cortical columns. These
connections are believed to link functionally similar regions,
as suggested by the observation that cytochrome-oxidase
blobs in the monkey primary visual cortex (V1) are preferentially
connected to blobs and interblobs to interblobs. To better
understand the precise relationship between horizontal
connections and blobs, we intracellularly labeled 20 layer
2/3 pyramidal neurons in tangential living brain slices
from V1 of macaque monkeys. The locations of each cell
body and the cell's synaptic boutons relative to blobs
were quantitatively analyzed. We found evidence for two
cell types located at characteristic distances from blob
centers: (1) neurons lacking long-distance, clustered axons
(somata 130–200 [mu]m from blob centers) and (2)
cells with clustered, long-distance axon collaterals (somata
<130 [mu]m or >200 [mu]m from blob centers). For
all cells, synaptic boutons close to the cell body were
located at similar distances from blob centers as the cell
body. The majority of boutons from cells lacking distal
axon clusters were close to their cell bodies. Cells located
more than 200 [mu]m from blob centers were in interblobs
and had long-distance clustered axon collaterals selectively
targeting distant interblob regions. Cells located less
than 130 [mu]m from blob centers were found within both
blobs and interblobs, but many were close to traditionally
defined borders. The distant synaptic boutons from these
cells were generally located relatively near to blob centers,
but the neurons closest to blob centers had synaptic boutons
closer to blob centers than those farther away. There was
not a sharp transition that would suggest specificity for
blobs and interblobs as discrete, binary entities. Instead
they appear to be extremes along a continuum. These observations
have important implications for the function of lateral
interactions within V1.